While some preterm births have known causes such as infections, a large proportion of them have no known cause.

Muglia says research on human pregnancy using animal models has been hampered by the fact that human physiology in pregnancy is unique.

So the researchers took an evolutionary perspective instead and reasoned that the genes involved in birth timing would have evolved fast given the awkward nature of human birth.

Compared to other species, humans babies have a relatively big brain and head, while human mothers have a relatively narrow birth canal.

Muglia and colleagues believe that human pregnancy quickly evolved to optimise the size of infants at birth to reduce the chance mother and baby die during birth.

"Humans have the shortest gestation period relative to their brain and body size of any primate species," says Muglia.

"That trend has been strongest along the most recent branch leading to modern humans."

Comparing genomes

Muglia and colleagues set out to test their idea that genes involved in birth timing would have evolved quickly, compared to other primates and mammals, to achieve the shortest possible gestation period.

They then looked at how 9000 variants of these genes, called SNPs (single-nucleotide polymorphisms) were associated with preterm birth in humans.

Their initial analysis of 300 Finnish women pointed the finger at a gene that codes for the receptor for follicle stimulating hormone (FSH).

They also found similar trends in African American women.

"It was surprising because we'd been studying this in other model organisms for many years and had not really identified this pathway as being one that would likely influence human pregnancy outcomes," says Muglia.

It's especially a surprise because FSH has no known association with labour.

"This is a gene we would not have identified thinking about classic physiology of birth timing," says Muglia.

Maglia also studied the association in Caucasian American and Hispanic American women. While he found a similar trend, it was not statistically significant. He believes this is most likely due to the small size of these samples.

He says the next step is to see if the association is found in larger populations and to try and work out how this gene can influence birth timing.

If the association holds then Muglia says it could lead to a genetic screening test to identify women at risk of preterm birth.

If the mechanism by which the gene variants influence birth timing is understood, drugs to prevent preterm birth could also be developed, says Muglia.

'Interesting but unconvincing'

"I think the most exciting thing from it is the idea of looking for the cause of preterm birth by comparing the evolutionary development of humans with other primates. I think that's exciting and novel."

But he does not believe the findings in themselves will give a huge advance in our understanding of preterm birth.

He says the association with the FSH receptor gene was not clear cut in American women.

"If this is genuinely something that is evolutionarily conserved, and that's the argument, then you would expect it to be in all humans," he says.

Smith is not convinced by Muglia's argument that the lack of statistical significance of the association found in some groups of women is due to small sample sizes.

"It's obviously not a dramatic effect if it can't be picked up in small groups," says Smith.

And he says while FSH is important in establishing pregnancies in the first place, it is difficult to see how it is involved in the timing of birth.

But he says it's possible that by examining the function of FSH, and the gene that codes for its receptor, more closely we may find a new understanding of how human birth works.

"It's a tantalising possibility that there are new pathways that we don't yet understand that are involved in birth," says Smith.